The field of this invention relates to pump through ball seats used to build downhole pressure to actuate tools and more particularly to ball seats for use with liner hangers which must accommodate subsequent passage of wiper plugs during liner cementing or a larger ball for further downhole operations.
Downhole operations frequently involve the need to build up pressure to set a tool and/or to release from a tool. After the setting and release occurs, there is a need for access downhole. In the past ball seats have been used in combination with a ball or balls dropped from the surface to provide a way to close a tubular temporarily to allow for the requisite pressure buildup. The ball seats have to serve conflicting functions. They must be sturdy enough to withstand large differential pressures for a sufficient time to set the tool. They must cleanly release the ball to allow for subsequent objects such as wiper plugs or another, bigger, ball to pass through the spent ball seat with minimal pressure drop. They must be relatively easy to mill out of the way to accommodate subsequent downhole operations.
Yet another problem is the potential to over pressure the formation below as the requisite pressure on the ball has been built up and needs to be released. In the past, this problem has been addressed by using a reduced shock mechanism as part of the ball seat design. As shown in U.S. Pat. No. 6,079,496, the ball seat is movably mounted with the landing collar and pressure buildup on the ball moves the ball seat to reduce the volume of a variable volume cavity whose outlet is restricted. The restrictor, in turn, regulates the flow out of the cavity, which forces the ball seat to move at a predetermined rate, to reduce shock on the formation below. This Patent also teaches the use of non-metallic materials to facilitate milling out of the landing collar. Millout must occur because the ball seat assembly is designed to remain downhole with the liner being set and cemented.
Other prior designs have focused on construction of the ball seat. Some designs used segmented collets which shifted longitudinally under pressure with a ball on the seat formed by the segmented collets until a recess was reached allowing the segmented collets to spread and the ball to pass. Some examples of the segmented collet design are U.S. Pat. Nos. 5,244,044; 4,893,678; 4,823,882; 4,292,988; 3,220,481. Of these, U.S. Pat. No. 4,292,988 is most notable because it also has a provision to regulate the movement of the ball seat after its securing shear pin is broken to reduce shock. Another design involved a solid ball seat which expanded when moved to an unsupported position to let the ball pass. Some examples of this design are U.S. Pat. Nos. 4,520,870; 4,510,994; 4,114,694; 3,090,442; 4,862,966 and 6,155,350 (which also incorporates a controlled release pressure feature). Still other designs contemplated plastic deformation of the seat or controlled breakage along scoring of the seat to allow the ball or balls to be pumped through. Examples of this variation are U.S. Pat. Nos. 5,146,992 and 5,960,881.
Some of the drawbacks of the prior designs are addressed as the objectives of the present invention. The ball seat assembly is removable with the setting tool and running string so that it does not need to be milled out subsequently. The ball seat is firmly supported by segmented dogs held together with roll pins and disposed on the back side of the solid frusto-conically shaped ball seat. The problem of erosion of the ball due to rapidly moving fluid that could leak past segmented collets forming the ball seat is eliminated with the new ball seat design.
Another drawback of prior designs which used solid ball seats, such as U.S. Pat. Nos. 5,146,992 and 5,960,881 is eliminated by the present invention. In the past after an initial ball was pushed through the seat, subsequent balls would require high pressures to clear through the ball seat because of the point of contact made with the ball seat by the bigger ball. This was undesirable as it was advantageous to get the next and larger ball through the seat at low pressure differentials to expedite the next downhole operation and to avoid setting off relief devices built into such subsequent balls. These and other advantages of the present invention will become more apparent to those skilled in the art from a review of the description of the preferred embodiment, described below.
A removable ball seat assembly is disclosed. It features a solid ball seat backed up by segmented dogs pinned to each other and mounted under the ball seat. Upon actuating a downhole tool with fluid pressure applied to a ball on the seat, the pressure is increased and the ball and seat move at a regulated rate. The dogs reach a recess and the ball moves through the seat. Subsequent, larger balls can pass through the seat, with the dogs in the recess, at much smaller pressure drops than the original ball.